The effects of CNS treatment (cranial irradiation and intrathecal chemotherapy) on the cognitive and perceptual functioning of children with leukemia are investigated. One study compares a group of leukemic children in remission with a matched group of siblings on psychological tests (the Wechsler and Bender-Gestalt). These groups are followed-up 12-15 months after the initial assessment. A sample of leukemia patients who have not received CNS treatment and their sibs and a sample of cystic fibrosis patients and their sibs are also being assessed in order to control for effects other than CNS treatment. A second study is prospective. Children in remission from leukemia are evaluated on the Wechsler and Bender-Gestalt. They are tested just prior to prophylactic CNS treatment and one year later to determine possible brain changes resulting in learning disabilities and diminished intellectual functioning. The healthy siblings of the leukemics receiving CNS treatment, who are being studied retrospectively, are used as a control group to check testing effects. A five year longitudinal study investigating the neuropsychological correlates of CNS treatment of leukemic cases at various Children's Hospitals is being undertaken.